Stringent emissions legislation in Europe and North America is driving the implementation of new exhaust after-treatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Exhaust after-treatment technologies are currently being developed that will treat nitrogen oxides (NOx) under these conditions. One of these technologies includes a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust NOx to produce nitrogen (N2) and water (H2O). This technology is referred to as Selective Catalytic Reduction (SCR).
Ammonia is difficult to handle in its pure form in the automotive environment, therefore it is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea (CO(NH2)2). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue. The urea is delivered to the hot exhaust stream and is transformed into ammonia prior to entry in the catalyst.
The preferred transformation of urea is through hydrolysis and thermolysis to ammonia:                Thermolysis: CO(NH2)2→NH3+HCNO        Hydrolysis: HCNO+H2O→NH3+CO2         
However, urea is also known to break down into other compounds under certain conditions. Literature indicates that under pyrolysis at temperatures up to 350° C., the urea decomposes into biuret, cyanuric acid, ammeline, ammelide, and melamine. See “Thermal decomposition (pyrolysis) of urea in an open reaction vessel”, Schaber et al, Thermochimica Acta 424 (2004) 131-142. FIG. 1 show plots of the mass decomposition as a function of temperature. Such organic compounds can build-up deposits on exposed surfaces that can interfere with the proper operation of the RDU. These deposits can be found on the exhaust pipe walls, but also on the injector mounting boss surfaces as well as the injector flange surface.
FIG. 2 is a cross-sectional view of a conventional RDU, generally indicated at 10, having an injector flange 11 with a conical outlet 12. FIG. 3 is an enlarged view of the conical outlet 12 of FIG. 2. The initiation point A of deposit formation on the conical outlet 12 is characterized by a sharp break with an angle of 37.22 from the injector axis B. The deposits mechanism is characterized by the injection of urea into the exhaust, and then the return of the urea to the initiation point after the main injection has been completed.
Thus, there is a need to provide an RDU that is resistance to the build-up of deposits.